My curved spider sags...suggestions?

Now that Ive been using my new build, it does not hold collimation horizon/zenith as well as I'd initially thought. A thorough testing revealed that the dual struts and UTA are rock-solid, no shifting. The problem is the curved spider. It sags as the scope is raised/lowered.

I wish to keep the curved spider design. Should I remake it with wider metal? I used a steel ruler for this first one, which you can see in the pic below:

No pic there. We have arched vanes that you can't get commercially for 2.6" secondaries. It took some tweaking but we finally figured out how to make them and they have absolutely no give/sag (if yer interested to make yer own).

Was layin on the basement floor starin at the ceilin and came up with an arched whose tabs would fold inward and overlap to form a rigid focuser mounting plate. We would use what would be left of the 1/32" aircraft aluminum sheet (that is the skin wrap over the 2dary cage), 2-ply it and laminate with JB Weld . The aluminum has too much spring to wanna keep its curve. Drilled the templates together such that the spring tension of the aluminum would go down (by half?) and the JB Weld would form rivets. It's tapered, too, as per our original plan but later to just a flare from 2.25" to 3" at the nodes. (I call the ends "tapered chords" because that's what Art Bionci(?) [dancin Snoopy] called them in his CAD plans, archived somewhere here at CN. Wish I could find that thread, but once we saw it we built with better confidence. Decided to flip one template over the other to slightly offset the holes. This gave us JB Welt rivets with tabs, and these held the arch beautifully.

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and here. And at 1/16th inch, the vane looks like wire thru the focuser. When we do it again we'll probly use steel with greater strength/less spring. Also, the arch seems the simplest and best way to mount with confidence. Either way, our laser never leaves the primary center spot.

Notice that on my spider, the mass of that 1.83 secondary is fairly distant from where the vanes attach. Wondering if that imbalance is the prob and...if so...if I could balance it via washers at the opposite end...or am I just plain dumb????

Is that ONE screw per side for both the vane node AND its mounting bracket? As for the mass and distance, these can't change, but you would help yerself by usin two over-under screws to mount with versus one. And whether for this you'll need a wider vane, at least at the tabs, then so much the better. It also looks like yer at about 1/16" which is plenty thick and strong for steel, and better than we have, so widening it would give better stiffness too while also givin room for sufficient 2-screw over-under mounting against that direction of flex/sag. Also, are you really at 180 degrees? Looks like yer gonna bring the ends in closer together, and this will add rigidity too when you help it with two screws torquing against the direction of sag.

Sure enough, there is but one screw per side per bracket; but they are cyano glued as well. Ill be checking this but...the laser pattern moves up/down repeatably. The flex just has to be in the vane it seems...the ends are fixed pretty well. The vane is .055 thick.

The "node" end is bolted/glued to the mounting bracket...the bracket is affixed to the UTA via one bolt.

Get another flexible ruler bend it to the same shape as the first and then epoxy (laminate) it to the first one so you have a 2-ply spider. Attempting to keep the spider too thin becomes an obsession with some folks. Better to make it too stiff than not. You may need a 3rd thickness. On my curved spider, it must be forced into place by the screws (ie: it is like a compressed spring when installed). I haven't decided exactly how important it is to do this.

Get another flexible ruler bend it to the same shape as the first and then epoxy (laminate) it to the first one so you have a 2-ply spider. Attempting to keep the spider too thin becomes an obsession with some folks. Better to make it too stiff than not. You may need a 3rd thickness. On my curved spider, it must be forced into place by the screws (ie: it is like a compressed spring when installed). I haven't decided exactly how important it is to do this.

You can see my spider in the link below for the 6" F4 reflector.

My opinion anyway.

John

There ya go. Use two over-under screws at every mounting surface and draw those ends in a little, to 180 degrees at mirror edge. Also eliminates the cyano- , but if yer determined to add that then at least use Loctite heavy-duty builder's 2-part epoxy-resin. But better is to just use two screws thru slightly over-sized holes (can even use locking screws) for some adjustment room.

Here is a link to a page with a picture of the only curved spider I've found stable:http://www.obsession.../12.5/index.php
It's the second picture on the right side of the page down from the top.
It seems to resist sag quite well.

Here is a link to a page with a picture of the only curved spider I've found stable:http://www.obsession.../12.5/index.phpIt's the second picture on the right side of the page down from the top.It seems to resist sag quite well.

Terry, I can't help you with your curved spider woes (but seems several others can) ... however, I would like to compliment you on the exceptional craftsmanship that is evident in your instrument ... it is really a work of art, beautiful to look at. Well done, and thanks for sharing. Dave O

Terry, I can't help you with your curved spider woes (but seems several others can) ... however, I would like to compliment you on the exceptional craftsmanship that is evident in your instrument ... it is really a work of art, beautiful to look at. Well done, and thanks for sharing. Dave O

Dave, that is VERY kind of you to say. I truly appreciate it! And many thanks to all who have offered help!!!

This is what I've been doing for years. It seems to help by keeping the mirror from being cantilevered with respect to the tube attachment and of course the stiffness is helped by the flared support. YMMV of course.

Best,--Mike spooner

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Update: as some may know, this is a new build and hense, Im just getting to know it. Ive cut "the sag" by about 75% simply by tightening things up....Ive had to learn how tight to set things...was a bit too light on the tightening at first. So..its an f6 10"...at this time the laser spot as viewed against the Cateye style centermark on the primary (ie triangular) wanders aprox 1/8" or a tad less as I move horizon/zenith.
Should I persue this more or am I in an acceptable range at this point?

BTW there is zero movement at the spider atatchment points bracket/ota. So...that is not the issue.

I followed Clyde's instructions and made my similar professionally made Protostar curved spider work just fine in my 10 inch F6 tube Dob. Check out his spider in this link:http://www.nmsu.edu/...r2001/path.html